An LED drive circuit configured to cause a plurality of LEDs included in an LED array to emit light by applying a pulsating current in a full-wave rectified waveform or the like, which is obtained by rectifying an alternating-current voltage that is supplied from the commercial alternating-current power source, to the LED array in which the plurality of LEDs is connected in series is known. Further, it is known to cause a partial LED array to blink in accordance with the phase of a pulsating current by arranging a bypass circuit at a connection part that connects the partial LED arrays obtained by dividing an LED array in an LED drive circuit configured to apply a pulsating current to the LED array. Furthermore, it is known to select a partial LED array that is caused to emit light by detecting a current that flows through an LED array and controlling a bypass circuit in accordance with the detected current in an LED drive circuit.
FIG. 4 is a circuit block diagram of an LED drive circuit 100 configured to control a bypass circuit in accordance with a current that flows through an LED array.
The LED drive circuit 100 has a rectification circuit 107 to which an AC voltage is input from a commercial alternating-current power source 101, an LED array 110 including a first partial LED array 111 and a second partial LED array 112, a bypass circuit 120, and a current limiting circuit 130. The bypass circuit 120 includes a bypass FET 121 and a bypass resistor 122. The current limiting circuit 130 includes a current limiting FET 131 and a current limiting resistor 132. The first partial LED array 111 and the second partial LED array 112 are connected in series in such a manner that the first LED array 111 is located in the previous stage of the second partial LED array 122. The bypass FET 121 and the current limiting FET 131 are each a depletion FET. The bypass resistor 122 and the current limiting resistor 132 are each a resistance element that detects a current.
The LED drive circuit 100 further has a fuse 103 that is connected to the commercial alternating-current power source 101, and an overvoltage suppressor (TVS) 105. The rectification circuit 107 has a diode bridge and outputs a full-wave rectified waveform voltage Vrect. A V-terminal (node n4) of the rectification circuit 107 is at the ground level.
FIGS. 5A and 5B are diagrams each illustrating the operation of the LED drive circuit 100, and FIG. 5A is a diagram illustrating one cycle of the full-wave rectified waveform voltage Vrect and FIG. 5B is a diagram illustrating a light emitting current IL that flows through the bypass resistor 122. In FIG. 5A, the horizontal axis represents a time t, the vertical axis represents a voltage V, and the curve in FIG. 5A represents the full-wave rectified waveform voltage Vrect. In FIG. 5B, the horizontal axis represents the time t, the vertical axis represents a current I, and the curve in FIG. 5B represents the light emitting current IL that flows through the bypass resistor 122. Each of t11 to t15 illustrated along the time axis in FIG. 5A corresponds to each of t11 to t15 illustrated along the time axis in FIG. 5B.
During the period of time t11, the full-wave rectified waveform voltage Vrect is lower than a threshold voltage Vth1 of the first partial LED array 111, and therefore the light emitting current IL does not flow through the LED array 110. The threshold voltage Vth1 of the first partial LED array 111 is the total sum of forward voltage drops of the LEDs connected in series in the first partial LED array 111.
During the period of time t12, the full-wave rectified waveform voltage Vrect increases and the full-wave rectified waveform voltage Vrect becomes higher than the threshold voltage Vth1 of the first partial LED array 111, and therefore a first light emitting current IG1 flows through the first partial LED array 111. However, during the period of time t12, the full-wave rectified waveform voltage Vrect is lower than the total value of the threshold voltage Vth1 of the first partial LED array 111 and a threshold voltage Vth2 of the second partial LED array 112, and therefore a current IG2 does not flow through the second partial LED array 112. The first light emitting current IG1 does not flow via the second partial LED array 112 but flows into the rectification circuit 107 via nodes n1 and n2, the bypass FET 121, and the bypass resistor 122. During the period of time of t12, the voltage drop due to the bypass resistor 122 makes constant a gate-source voltage Vgs of the bypass FET 121, and therefore the bypass FET 121 performs a constant current operation. During the period of time t12, the light emitting current IL that flows through the bypass resistor 122 is equal in magnitude to the current IG1 that flows through the first partial LED array 111. During the last period of time of the period of time t12, the full-wave rectified waveform voltage Vrect is slightly higher than the total value of the threshold voltage Vth1 of the first partial LED array 111 and the threshold voltage Vth2 of the second partial LED array 112. Since the full-wave rectified waveform voltage Vrect is slightly higher than the total value of the threshold voltage Vth1 of the first partial LED array 111 and the threshold voltage Vth2 of the second partial LED array 112, the current IG2 flows through the second partial LED array 112. During the last period of time of the period of time t12, the light emitting current IL that flows through the bypass resistor 122, which has the total value of the current IG1 and the current IG2, is a current I12 and is almost constant, since the amount of decrease in the current IG1 accompanying the increase in Vgs of the bypass FET 121 is almost equal to the amount of increase in the current IG2.
During the period of time t13, due to the increase in the current IG2, the voltage drop due to the bypass resistor 122 becomes large, Vgs of the bypass FET 121 becomes lower than a cutoff voltage, and the bypass FET 121 is cut off. During the period of time t13, the current limiting FET 131 performs the constant current operation, since the voltage drop due to the current limiting resistor 132 makes constant the gate-source voltage Vgs of the current limiting FET 131. During the period of time t13, the light emitting current IL that flows through the bypass resistor 122 is equal to the current IG2 that flows through the second partial LED array 112 and becomes a current I11. The current I11 is the upper limit value of the current that flows through the current limiting circuit 130 including the current limiting FET 131 and the current limiting resistor 132. During the second half of the period of time t13, and the periods of time t14 and t15 during which the full-wave rectified waveform voltage Vrect drops, the light emitting current IL decreases in the opposite manner to that during the periods of time t11 to t13.
In the LED drive circuit 100, the LED array 110 is divided into a plurality of partial LED arrays: the first partial LED array 111 and the second partial LED array 112. The bypass circuit 120 including the bypass FET 121 and the bypass resistor 122 is connected to the node n2 that is the connection part between the first partial LED array 111 and the second partial LED array 112. The bypass FET 121 of the bypass circuit 120 is cut off when the light emitting current IG2 that flows through the second partial LED array 112 becomes large and the gate-source voltage Vgs that is regulated by the voltage of the bypass resistor 122 becomes lower than the cutoff voltage. When the full-wave rectified waveform voltage Vrect, which is a pulsating current, is lower than the total value of the threshold voltage Vth1 of the first partial LED array 111 and the threshold voltage Vth2 of the second partial LED array 112, the LEDs of the first partial LED array 111 emit light, and the LEDs of the second partial LED array 112 do not emit light. When the full-wave rectified waveform voltage Vrect, which is a pulsating current, is equal to or higher than the total value of the threshold voltage Vth1 of the first partial LED array 111 and the threshold voltage Vth2 of the second partial LED array 112, the LEDs included both in the first partial LED array 111 and in the second partial LED array 112 emit light. The LED drive circuit 100 selectively causes the first partial LED array 111 and the second partial LED array 112 to emit light in accordance with the full-wave rectified waveform voltage Vrect.